Range-finding using light interference caused by a laser has long been used as a high-precision measurement method which does not disturb a measurement target because of noncontact measurement. Attempts have recently been made to use a semiconductor laser as an optical measurement light source in order to miniaturize an apparatus. A typical example uses an FM heterodyne interferometer. This apparatus can measure relatively long distances and has high precision. However, the apparatus uses an interferometer outside a semiconductor laser, and hence requires a complicated optical system.
There has also been proposed a measurement instrument using the interference (self-mixing effect/self-coupling effect) between output light from a laser and return light from a measurement target in a semiconductor laser. Such self-mixing/self-coupling type laser measurement instruments are disclosed in, for example, reference 1 (Tadashi Ueda, Jun Yamada, and Susumu Shito, “Range Finder Using Self-Coupling Effect of Semiconductor Laser”, 1994 TOKAI-SECTION JOINT CONVENTION RECORD OF THE SIX INSTITUTES OF ELECTRICAL AND RELATED ENGINEERS), Reference 2 (Jun Yamada, Susumu Shito, Norio Tsuda, and Tadashi Ueda, “Study of Compact Distance Meter by Self-Coupled Effect of Laser Diode”, Bulletin of Aichi Institute of Technology, Vol. 31 B pp. 35-42, 1996), and reference 3 (Guido Giuliani, Michele Norgia, Silvano Donati and Thierry Bosch, “Laser diode self-mixing technique for sensing applications”, JOURNAL OF OPTICS A: PURE AND APPLIED OPTICS, pp. 283-294, 2002).
According to a self-mixing/self-coupling type laser measurement instrument, since a semiconductor laser built in a photodiode has a combination of light-emitting, interference, and light-receiving functions, an external interference optical system can be greatly simplified. Therefore, a sensor unit comprises only a semiconductor laser and a lens, resulting in a reduction in size as compared with the prior art. In addition, a characteristic of this technique is that it has a range-finding range wider than that of the triangulation method.
FIG. 20 shows a complex resonator model of an FP type (Fabry-Perot type) semiconductor laser. Referring to FIG. 20, reference numeral 101 denotes semiconductor laser resonator; 102, a cleavage surface of a semiconductor crystal; 103, a photodiode; and 104, a measurement target. Part of reflected light from the measurement target 104 tends to return into the oscillation region. The small amount of light which has returned mixes with laser light inside the resonator 101. This makes the resonator operate unstably and causes noise (complex resonator noise or return light noise). Even return light of a very small amount relative to output light causes a noticeable change in the characteristics of the semiconductor laser. Such a phenomenon occurs not only in a Fabry-Perot type (to be referred to as an FP type hereinafter) semiconductor laser but also in other types of semiconductor lasers such as a vertical cavity surface emitting type semiconductor laser (to be referred to as a VCSEL) and a distributed feedback type semiconductor laser (to be referred to as a DFB laser).
Letting λ be the oscillation wavelength of the laser and L be the distance from the cleavage surface 102 near the measurement target 104 to the measurement target 104, when the following resonance condition is satisfied, return light and laser light inside the resonator 101 intensify each other to slightly increase the laser output:L=nλ/2  (1)where n is an integer. It is possible to satisfactorily observe this phenomenon, even if scattered light from the measurement target 104 is very weak, because the apparent reflectance in the resonator 101 of the semiconductor laser increases to produce an amplifying effect.
A semiconductor laser emits laser beams having different frequencies in accordance with the magnitude of an injection current, and hence allows to perform direct modulation of the oscillation frequency using an injection current without requiring any external modulator. FIG. 21 is a graph showing the relationship between the oscillation wavelength of the semiconductor laser and the output waveform of the photodiode 103 when the oscillation wavelength changes at a given constant rate. When L=nλ/2 indicated by equation (1) is satisfied, the phase difference between return light and laser light inside the resonator 101 is 0° (in phase), and the return light and the laser light inside the resonator 101 intensify each other most. When L=nλ/2+λ/4, the phase difference becomes 180° (in opposite phase), the return light and the laser light inside the resonator 101 weaken each other most. For this reason, as the oscillation wavelength of the semiconductor laser changes, the intensity of the laser output alternately and repeatedly increases and decreases. Detecting the laser output at this time by using the photodiode 103 provided for the resonator 101 will obtain a stepwise waveform with a predetermined period as shown in FIG. 21. Such a waveform is generally called an interference fringe.
Each one of the stepwise waveform components, i.e., the interference fringe components, is called a mode hop pulse (to be referred to as an MHP hereinafter). MHP is a phenomenon different from the mode hopping phenomenon to be described later. Assume that the distance to the measurement target 104 is represented by L1 and the number of MHPs is 10. In this case, when the distance decreases to a distance L2 which is half of the distance L2, the number of MHPs becomes five. That is, when the oscillation wavelength of the semiconductor laser is changed in a predetermined time period, the number of MHPs changes in proportion to the measurement distance. Therefore, detecting MHPs by using the photodiode 103 and measuring the frequency of the MHPs can easily measure the distance. Note that the mode hopping phenomenon unique to an FP type semiconductor laser is a phenomenon in which an oscillation wavelength has discontinuous portions in accordance with a continuous increase/decrease in injection current. When the injection current increases and decreases, the oscillation wavelength exhibits slight hystereses.